This invention relates to improvements in or to an electromagnetic spring-wound clutch.
An electromagnetic spring-wound clutch is well known which comprises: an input rotary member and an output rotary member arranged concentrically with each other; an armature member disposed for magnetically frictional engagement with one of the two rotary members; an electromagnetic coil operable when excited to cause the magnetically frictional engagement of the armature member with the one rotary member; and a coil spring having one end engaging the armature member and the other end with the other rotary member, respectively, and disposed for gripping engagement with the two rotary members. When the armature member is magnetically attracted and drawn to the above one rotary member, the coil spring is wound up into gripping engagement with the two rotary members to cause driving coupling between them.
More specifically, as known e.g. from Japanese Patent Publication No. 56-54493, a typical electromagnetic spring-wound clutch of this type comprises: a retainer member adapted for coupling to a framework of an apparatus for use with the clutch; a first rotary member (hub) rotatably supported on the retainer member; a field core/electromagnetic coil assembly accommodated within the first rotary member and supported by the retainer member; a second rotary member arranged at one end of the first rotary member and having a tubular boss adapted for coupling to a rotary shaft of the above apparatus; an armature member interposed between the two rotary members; and a coil spring having one end engaging with the second rotary member and the other end with the armature member, respectively.
According to this conventional electromagnetic spring-wound clutch, the first rotary member or hub is fitted, via a radial ball bearing or the like, on the retainer member which is arranged around the rotary shaft of the apparatus. Further, the hub is adapted to be coupled to a driving means or a driven means via a pulley formed on its outer periphery or a like means. Still further, the hub is formed therein with an internal space accommodating the field core/electromagnetic coil. Therefore, the hub has a very large radial size. In addition, to increase the torque transmission efficiency of the electromagnetic spring-wound clutch, the sizes of the electromagnetic coil and the field core have to be increased, and also the area of engagement of the coil spring with the first and second rotary members on which the former is grippingly wound up has to be increased. In the former case, the first rotary member or hub has a limited space for accommodating the field core/electromagnetic coil assembly having an increased size, and in the latter case, an increase in the above engaging area will necessitate an increase in the axial size of the clutch.
However, in recent years electromagnetic spring-wound clutches of this kind have been widely used in electronic copying machines, electronic computers and various automated apparatus. It is a general recent tendency that those apparatus become more compact in size. To this end, electromagnetic spring-wound clutches for use with these apparatus are required to satisfy two incompatible requirements at the same time, that is, they should have high torque transmission efficiency and their sizes should be as small as possible.
On the other hand, electromagnetic spring-wound clutches of this kind are arranged such that a coil spring is wound on two rotary members having exactly the same outer diameter or almost the same outer diameter for gripping engagement therewith to achieve driving coupling therebetween. A period of time required for obtaining this driving coupling through gripping winding of the spring determines the coupling period of time of the clutch that is a period of time from the start of energizing the coil to the start of torque transmission.
This coupling period of time is determined by the clearance between the inner peripheral surface of the coil spring in a non-gripping state and the outer peripheral surface of the gripping engaging portions of the two rotary members. Therefore, in designing a clutch, the above clearance is set to such a value as obtains a desired coupling period of time.
Therefore, a completed clutch operates with a constant coupling period of time irrespective of load conditions of apparatus for use with the clutch, and therefore it cannot always satisfy requirements of a variety of apparatus intended for use with the clutch.